Amp-hour (Ah)

Amp-hours measure how much current a battery can supply over time, and it is the number to compare when sizing a house battery bank, not the physical size or group number. A 100Ah battery can, in theory, supply 100 amps for one hour or 5 amps for twenty hours, though real-world output varies with load and temperature.

How much of that rating you can actually use depends on chemistry. Flooded lead-acid and AGM batteries are typically kept above 50 percent state of charge to avoid shortening their lifespan, so a 100Ah lead-acid battery offers roughly 50Ah of practical use. LiFePO4 batteries can be discharged to 80 to 100 percent of their rating regularly without the same wear penalty.

The most common error is comparing amp-hour numbers across chemistries without adjusting for usable capacity, which makes lead-acid banks look more competitive with lithium than they really are. When sizing a bank, estimate daily amp-hour draw from your appliances first, then work backward to the usable capacity you need.